It is known that permanent reconstruction of a bone joint which has been malformed from birth, or as a result of disease, or accidental injury, may be achieved by the substitution of the afflicted portion(s) of the joint by an artificial implant constructed of a biocompatible material. It is desirable that any replacement joint should imitate the natural articulation of the healthy joint.
One method of replacing the hip joint involves the use of a stemmed femoral prosthesis adapted to be inserted and retained within the medullary canal of the femur after removal of the natural femoral head. The socket function of the replacement joint is performed by an acetabular cup implant which defines a spherical recess adapted to receive the ball end of the femoral prosthesis. Dislocation of the femoral ball is prevented by the soft tissue structures and the natural stability of the spherical recess. However, it is possible for the femoral head to be removed from the acetabulum, thus causing a dislocation for various reasons including impingement, soft tissue laxity, or component malposition.
Most known hip prostheses require both the femoral and acetabular components to be fixed rigidly into their corresponding bones, either by a tight press-fit, cemented, or screw attachment. Without the ability to rotate the femur around its longitudinal axis in a controlled manner, due to muscle configuration and surrounding tissue, impingement of known types of hip prostheses is easily possible. Known hip implants include range-of-motion regions posing the danger of aggressive dislocation (by leverage) or implant damage if the femur were to be rotated into an unsuitable orientation.